This invention pertains to a method of applying a hot melt adhesive foam, and to products manufactured by this method.
Historically, a roll coater or extruder has been used to apply a hot melt adhesive to the surface of a substrate. And it is also known to spray a hot melt adhesive on the surface of a substrate. The main constituent of a hot melt adhesive is usually a thermoplastic resin. However, a hot melt adhesive can also include plasticizers, stabilizers, and adhesive resins as additives in small quantities, as necessary. But unlike coating materials, a hot melt adhesive includes no diluent. Therefore, the viscosity of a hot melt adhesive is comparatively high, and its coagulating power is also strong.
When a hot melt adhesive is sprayed the sprayed particles are comparatively large. Specifically, the diameter range of the hot melt adhesive spray particles can vary widely, e.g., between 50 and 5,000 microns even at a viscosity of 1000 cps, which is comparatively low for a hot melt adhesive. The fact that the spray particles are so large and vary so widely in size leads not only to waste of adhesive material in the manufacturing process, but also to low-quality products as a result of weak or nonuniform adhesion. In the case of foils and films, the surface of the foil or film may become rough when it is bonded to a substrate through use of a sprayed hot melt adhesive, thereby resulting in the loss of an attractive appearance. Further, the hardened hot melt adhesive is rough to the touch. For these reasons, the spraying of hot melt adhesives has been adopted only in special areas.
The present invention pertains to a method for applying a foam of hot melt adhesive by the use of an air spray, and to products manufactured by this method that incorporate the hot melt adhesive foam as a bonding agent. The invention, which makes efficient use of the properties of foamable hot melt adhesives, is aimed at solving the defects of the products to which hot melt adhesives have been applied by conventional spraying methods. In the invention, a hot melt adhesive foam is applied to the surface of a first substrate by means of hot air spraying. Importantly, the spray conditions are regulated such that an adhesive film of generally spider web type configuration, i.e., an open weave or open mesh film with significant gaps or openings throughout its surface area, is formed on the first substrate. A second substrate can then be bonded to the first to form a laminate product. Alternatively, and before the second substrate is bonded to the first, a powder, e.g., a hygroscopic powder, can be deposited within the open mesh areas of the adhesive film's spider web configuration in order to form a laminate product with a special enhanced characteristic, e.g., moisture absorbency.
The foamed hot melt adhesive method of this invention not only retains its primary property of adhesivity but also provides the following merits in the adhesion process: a reduction in the amount of adhesive material used as a result of a decrease in specific gravity; a lowered apparent viscosity under high pressure in the applicator; a decreased fluidity after application; a thinner adhesion layer and an expanded adhesion area after adhesion of the substrates under pressure; a shortened set time; improved adhesion when used on porous or rough surfaces; a reduced surface heat capacity which prevents surface heat deformation when applied to thin sheets and other materials; an increase in the feasibility of low temperature applications because of the relative rapid cooling of the substrate surface; improved sealability; improved softness and touch for the finished laminate product; light weight; increased open time in the manufacturing process; and reduced total cost.